When the reference signal of the fractional phase locked loop has a frequency which is too low compared to the frequency of the output signal provided by the voltage-controlled oscillator of the phase locked loop, the phase locked loop may not properly filter the noise resulting from the delta-sigma modulation. The noise, therefore, may appear in the output spectrum of the signal provided by the voltage-controlled oscillator.
This is the case, for example, in the BLE (Bluetooth Low Energy) standard, where the signal provided by the oscillator of the loop has a frequency of 2.4 GHz and the reference signal of this loop has a frequency of 16 MHz.
It has been observed that one solution involved doubling the frequency of the reference signal.
A classical solution for obtaining a double frequency consists in using the phase locked loop with two phase comparators operating respectively on the rising edges and the falling edges of the reference signal instead of a single phase comparator (phase frequency detector).
However, such architecture results in the presence of many contributions to the phase noise. Furthermore the signal of 32 MHz so produced is not usable by other devices that might be present in the integrated circuit incorporating this fractional phase locked loop with double phase comparator.